The increasing use of MIDI and music synthesis capabilities in applications such as personal computer multimedia, Karaoke, and low cost musical instruments is fueling the demand for high performance music synthesis systems.
FIG. 1 depicts a synthesis system architecture 100 common to many modern cost-effective wavetable synthesizer implementations. A plurality of processor units are implemented in this instrument, where specialized tasks are assigned to each processor unit to realize high speed multi-tasking data processing. The configuration includes a synthesis processing unit 102, in which the synthesizing arithmetical operations are mainly performed, a microprocessing unit 104, that controls the synthesis processing unit 102 and also performs slow synthesizing operations, an input/output-unit 106 (I/O-unit) for data exchange with external peripherals, e.g. computers, (MIDI) keyboards, via interfaces, a memory management unit 108 for data exchange with external memory (DRAM, SRAM, ROM, floppy drive, etc.), and a clock unit 110 for providing a clock signal and reset signal.
This approach utilizes a specialized synthesis DSP core 102 for the sample processing tasks which directly generates synthesized voices, and a general-purpose microprocessor 104 to implement the command parsing and control tasks. This allows the DSP core 102, which must perform a limited number of processing tasks repetitively and very efficiently, to be optimized for the music synthesis task. By implementing only the instructions and capabilities needed for specific synthesis algorithms, and by adding specialized hardware for synthesis-specific functions, the synthesis DSP 102 performance can be improved.
According to the above-mentioned configuration, the time-critical functions are realized in the specialized synthesis DSP 102, where repetitive operations on one set of tone sample data could be performed. This kind of electronic musical instrument is disclosed in "A Music Synthesizer Architecture which Integrates a Specialized DSP Core and a 16-bit Microprocessor on a Single Chip", presented at the 98th Convention of the Audio Engineering Society, Feb., 25, 1995 by Deforeit and Heckroth. However, in the publication, no teaching is given on how to handle positive or negative overflows that may occur on performing arithmetic operations.
A computer system handling positive and negative overflow resulting from arithmetic operations of a two's complements adder is disclosed in U.S. Pat. No. 5,448,509. In the case of positive and negative overflows, the result of the two's complement adder is replaced by predetermined maximum and minimum values, respectively. The predetermined maximum and minimum values depend on the predetermined assignment of signs to the operands and the predetermined number of bits per operand. This limitation of the range of values produces a straight clipping of exceeding values. For example, when the sum of two positive continuous functions exceeds the predetermined maximum value, discontinuous function will result, as depicted in FIG. 2A.
Also, U.S. Pat. No. 5,381,356 discloses a technique to handle overflow for use in a digital filter. In this case, the polarity of the result of the two's complement adder is inverted at the occurrence of an overflow. This means, for example, that during summation of a constant signal and a sinusoidal signal using this adder, a jump of the resulting value will occur, when the resulting value exceeds the maximum value (an overflow event), as depicted in FIG. 2B.
As described above, a conventional digital musical tone forming device includes one or more CPUs for performing specialized operations on tone sample data, where occasionally a digital overflow may result from the operation as the digital sample data values are limited by the available number of bits per digital number representing a range of valid integer numbers. Thus, either the overflow is handled in a way given above, producing abrupt discontinuities in the time behavior of the tone data synthesis, resulting in distortions during the reproduction of these tone data (e.g. by speakers), or the initial tone data values are limited to a fraction of the available number of bits being operated at, so that no overflow will occur during tone synthesis, however substantially limiting the dynamic range of tone reproduction. The above is also true for other audio systems transferring or processing digital tone data.
In other applications such as process control systems or fast calculating systems, the occurrence of an overflow may cause an unwanted or even dangerous system failure. Furthermore, the abrupt or discontinuous clipping of digital process parameters or numerical data according to the conventional art may cause serious instabilities in process control or numerical model calculations. On the other hand, a software implementation of other clipping modes slows down the overall calculation speed of the system.